1. Field of the Invention
The present invention relates to a system for reproducing from a record data-carrying signals stored upon a substrate the surface of which, along a rectilinear or curvelinear track, exhibits irregularities in the form of depressions or projections, which correspond with the time variation in the signals carrying said data. This time variation is optically detected during the scanning of the track, by the effect exercised by these surface irregularities upon the optical diffraction of a concentrated read-out beam which converges onto the engraved surface of the record.
The invention likewise relates to a record or data-carrier appropriate to said method of recording by virtue of depressions or projections, and to a reproducing device which makes it possible to effect optical read-out of the signals stored in relief form at the surface of said carrier.
2. Discussion of Background
In the known methods of reproducing signals which are stored in the form of superficial deformations of the surface of a record, for read-out purposes, use is made of a stylus which displaces in a groove imparting to the stylus lateral or depth displacements. The movements imparted by the modulation in the groove or track, to the tip of the stylus, are communicated to an electromechanical transducer which supplies a voltage proportional to the instantaneous velocity or amplitude of the tip of the read-out stylus. These methods have numerous drawbacks, the major of which are the wear in the stylus, in the carrier or substrate, the play back distortion and the difficulty of making the stylus assembly of the transducer follow the high frequency components which succeed one another at very close intervals.
Also known are methods of optically reading out a carrier carrying a photographically recorded track and exhibiting variations in optical density or in width, corresponding to the time variation of a signal. A difficulty with these systems, resides in the fact that the copies of the carrier are obtained by photochemical methods, more expensive than the die-stamping methods employed with ordinary sound record discs. Another difficulty of these optical methods which are based upon the projection of enlarged images, resides in the small depth of field of the objective lenses used to form the enlarged image of the track, and in the limitation imposed upon the resolving power, due to the phenomenon of diffraction. In order to overcome these difficulties, it has been proposed a method of holographic recording by which the carrier may be copied by a stamping operation, but this technique involves the use of relatively powerful coherent light sources since the diffraction efficiency of the recorded holograms is relatively poor. Hitherto, the major difficulty with systems for mechanically or optically scanning the carriers on which the signal is recorded, has consisted in maintaining a proportional relationship between the amplitude of the deformation recorded on the track, and the electrical signal furnished by the read-out equipment. This amplitude proportionality can be roughly satisfied if the stored signals are pulse time modulated waveforms taking the form of successive square wave envelopes containing transitions between two levels. The precise location of such transitions in time, serves to faithfully translate the information. This is particularly the case in telecommunications systems utilising frequency or phase modulation. There is nothing to prevent this principle from being applied to the field of optical recording with a view to simplifying the process of scanning and reading out a record wherein, the embossed pattern engraved corresponds to a purely time variation on the part of an alternating data-carrying signal.